Hubble Sequence - Group of Questions Last Modified: 2 years ago Hubble Sequence To print this group, add it to a test. Use the diagram below to answer the questions. Grade 9 Universe A. spiral with tightly wound arms spiral with loosely wound arms barred spiral with tightly wound arms barred spiral with loosely wound arms Grade 9 Universe elliptical, lenticular, and spiral galaxies spiral, lenticular, and elliptical galaxies. elliptical, irregular, and spiral galaxies spiral, irregular, and elliptical galaxies. Grade 9 Universe E3 is more elliptical in shape than E7 E7 is more elliptical in shape than E3 E3 has barred spiral arms and E7 does not E7 has barred spiral arms and E3 does not Grade 9 Universe H. M95 is a barred galaxy and Andromeda is not Andromeda is a barred galaxy and M95 is not M95 has more tightly wound spiral arms than Andromeda Andromeda has more tightly wound spiral arms than M95 Grade 9 Universe I. Edwin Hubble The Hubble Tuning Fork After he discovered what galaxies really were, Edwin Hubble became the first person to classify galaxies.

Astronomers use his system, called the "Hubble Tuning Fork," even today. First, Hubble divided the galaxies into two general categories: elliptical and spiral galaxies. Elliptical galaxies are shaped like ellipses, and spiral galaxies are shaped like spirals, with arms winding in to a bright center. Elliptical galaxies are classified by how round or flat they look. An E0 galaxy is very round and an E7 galaxy is very flat. In detail, the number after the "E" is determined by the galaxy's ellipticity - the ratio of the ellipse's major axis to minor axis. Galaxies with higher ellipticities have higher numbers. Hubble noticed that some spiral galaxies have a bright line, or bar, running through them. He called these galaxies "barred spiral galaxies." Galaxies with spiral arms, but without the bar, are just called "spiral galaxies." Spiral galaxies are further classified by how tightly their arms are wound. Type a galaxies have their arms wound very tightly and have large central bulges. Type c galaxies have very their arms would loosely and have small central bulges. Some galaxies are a transition type between the elliptical and spiral galaxies, labeled S0 on the tuning fork.

These are called "lenticular galaxies." Lenticular galaxies have a central bulge and a disk but no spiral arms. The third class of galaxies is irregular galaxies. Irregular galaxies are neither spiral nor elliptical, and can have any number of shapes. They are frequently the product of two galaxies colliding with each other, or at least affecting each other through the force of gravity. You can see why this diagram is called the Hubble tuning fork. Hubble believed that galaxies started at the left end of the diagram and evolved to the right. He called the elliptical galaxies "early galaxies" and the spirals "late galaxies." We now know that he was wrong: galaxies do not move down the forks of the diagram as they evolve. We know this because spiral galaxies rotate quickly (on an astronomical scale), while elliptical galaxies do not. There is no way that an elliptical galaxy could spontaneously begin rotating, so there is no way an elliptical galaxy could turn into a spiral galaxy. Although Hubble was wrong about his theory of galaxy evolution, his diagram provides a useful way to classify galaxies.

In fact, astronomers today still use his terminology: elliptical galaxies are still referred to as "early galaxies" and spirals as "late galaxies." Exercise 2: Go back to the galaxies you looked at in Exercise 1, shown again in the table below. Classify them on the Hubble Tuning Fork. Run Hubble sequence Hubble sequence : Hubble Scheme The Milky Way galaxy is not the only galaxy in the Universe. Edwin Hubble showed in the 1930's that most of the `nebula' seen in the sky are, in fact, external galaxies, systems composed of billions of stars well outside the Milky Way. Later work by Hubble demonstrated that galaxies fall into two rough classes based on the kind of energy they emit. Most galaxies belong to the class of normal galaxies, which have unvarying luminosity whose energy is accounted for by the stars and interstellar matter that they contain. A minority of external galaxies, about 2%, belong to the class of active galaxies which have unusually high and variable luminosities and their energy is largely non-stellar in origin. Normal galaxies have certain features in common.

Gravity holds the billions of stars together, and the densest region is in the center, called a core or bulge. Some galaxies have spiral or pinwheel arms. All galaxies have a faint outer region or envelope and a mysterious dark matter halo. The contents of galaxies vary from galaxy type to galaxy type, and with time . Almost all galaxy types can be found in groups or clusters . Many clusters of galaxies have a large, supergiant galaxy at its center which has grow by cannibalizing its neighbors. Our solar system is located in outer regions of a spiral galaxy we call the Milky Way. The nearest neighbor galaxy is Andromeda Galaxy (M31) . Above is a 3D plot of most of the Local Group of galaxies, the population of galaxies within 1000 kpc if the Milky Way. Clustering of dwarf satellite galaxies around the great Milky Way and Andromeda spirals can be seen. Once the existence of external galaxies was definitively established, it became customary to divide them into a number of classes based on their morphology, that is, their apparent shape and structure.

The goal of any classification system, either in biology or astronomy, is to reveal underlying physical properties which may lead to understanding the formation and evolution of galaxies. Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by American astronomer Edwin Hubble in 1926. In Hubble's scheme, which is based on the optical appearance of galaxy images on photographic plates, galaxies are divided into three general classes: ellipticals, spirals, and irregulars. Basically, the Hubble sequence asks the following questions: 1) is there any overall regularity or symmetry to the galaxy, 2) is the light concentrated in the center, 3) is there a disk, and 4) are there any spiral arms. Galaxy Colors: The various colors in a galaxy (red bulge, blue disks) is due to the types of stars found in those galaxy regions, called its stellar population. Big, massive stars burn their hydrogen fuel, by thermonuclear fusion, extremely fast. Thus, they are bright and hot = blue. Low mass stars, although more numerous, are cool in surface temperature (= red) and much fainter.

All this is displayed in a Hertzsprung-Russell Diagram of the young star cluster. The hot blue stars use their core fuel much faster than the fainter, cooler red stars. Therefore, a young stellar population has a mean color that is blue (the sum of the light from all the stars in the stellar population) since most of the light is coming from the hot stars. An old stellar population is red, since all the hot stars have died off (turned into red giant stars) leaving the faint cool stars. The bottom line is that the red regions of a galaxy are old, with no hot stars. The blue portions of a galaxy are young, meaning the stellar population that dominates this region is newly formed. Elliptical galaxies : Galaxies of this class have smoothly varying brightnesses, steadily decreasing outward from the center. They appear elliptical in shape, with lines of equal brightness made up of concentric and similar ellipses. These galaxies are nearly all of the same color: they are somewhat redder than the Sun. Ellipticals are also devoid of gas or dust and contain just old stars.

The Hubble sequence classifies ellipticals based on their shape as defined by the ratio of their semi-minor axis to their semi-major axis. Round ellipticals are classed E0, highly flattened ellipticals are classed E7. NGC 4881 All ellipticals look alike, NGC 4881 is a good example (NGC stands for New General Catalog). Notice how smooth and red NGC 4881 looks compared to the blue spirals to the right. A few ellipticals are close enough to us that we can resolve the individual stars within them, such as M32, a companion to the Andromedia Galaxy. Spiral galaxies : These galaxies are conspicuous for their spiral-shaped arms, which emanate from or near the nucleus and gradually wind outward to the edge. There are usually two opposing arms arranged symmetrically around the center. The nucleus of a spiral galaxy is a sharp-peaked area of smooth texture, which can be quite small or, in some cases, can make up the bulk of the galaxy. The arms are embedded in a thin disk of stars. Both the arms and the disk of a spiral system are blue in color, whereas its central areas are red like an elliptical galaxy.

The Hubble sequence classifies spirals on the amount of winding in their spiral arms, tightly wound spirals are type Sa, more open spirals are classed Sc. M100 Notice in the above picture of M100 from HST (the M stands for Messier, the French astronomer who named these galaxies), that the center of the spiral is red/yellow and the arms are blue. Hotter, younger stars are blue, older, cooler stars are red. Thus, the center of a spiral is made of old stars, with young stars in the arms formed recently out of gas and dust. NGC 4639 The bulge of NGC 4639 is quite distinct from the younger, bluer disk regions. NGC 1365 The Hubble sequence divides spirals into two classes, normal and barred spirals depending on the presence of a bar shaped bulge in the galaxy's center. NGC 1365 is a barred spiral galaxy. Note the distinct dark lanes of obscuring dust in the bar pointing toward the bulge. A close-up of the spiral arms shows blue nebula, sites of current star formation.

NGC 4826 (Black Eye Galaxy) NGC 4826 is a typical Sa type galaxy with very tight spiral arms. As spiral galaxies are seen edge-on the large amount of gas and dust is visible as dark lanes and filaments crossing in front of the bulge regions. M104 The Sombrero Galaxy (also known as M104 or NGC 4594) is an unbarred spiral galaxy in the constellation Virgo. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of a sombrero. Irregular galaxies : Most representatives of this class consist of grainy, highly irregular assemblages of luminous areas. They have no noticeable symmetry nor obvious central nucleus, and they are generally bluer in color than are the arms and disks of spiral galaxies. NGC 2363 NGC 2363 is an example of a nearby irregular galaxy. There is no well defined shape to the galaxy, nor are there spiral arms.

A close-up of the bright region on the east side shows a cluster of new stars embedded in the red glow of ionized hydrogen gas. Once thought to be an evolutionary sequence where ellipticals evolve into spirals, the Hubble sequence is now thought to reflect the formation process of a galaxy. Galaxies which form their stars quickly, use up all their gas and become ellipticals (whose shape is simply the angle we view it on the sky). Spirals are inhibited in their star formation and still have gas to form new stars today. Irregulars form their stars in an erratic manner, thus their irregular shapes. Star Formation in Galaxies: The one feature that correlates with the shape, appearance and color of a galaxy is the amount of current star formation. Stars form when giant clouds of hydrogen gas and dust collapse under their own gravity. As the cloud collapses it fragments into many smaller pieces, each section continues to collapse until thermonuclear fusion begins. Initial conditions for a galaxy determines its rate of star formation. For example, elliptical galaxies collapse early and form stars quickly. The gas is used up in its early years and today has the appearance of a smooth, red object with no current star formation.

Spirals, on the other hand, form slower, with lower rates of star formation. The gas that `fuels' star formation is used slower and, thus, there is plenty around today to continue to form stars within the spiral arms. Galaxies and the Universe - Galaxy Classification Galaxy Classification Galaxies show a vast range of forms, and faced with any such situation we would like to seek any underlying patterns. This allows a compact description of individual objects, and if we are fortunate will lead to physical understanding (the prototype system of this kind is the MK stellar classification). Galaxy classification has developed with this aim, from rough description of an image through distinctions among components with different astrophysical properties. It is important to keep in mind that existing schemes are based on the appearance of a galaxy at optical wavelengths (usually in blue light), over the range of surface brightness conveniently recordable from the Earth's surface. As such, these classifications are dominated by certain components of galaxies. There are crude hand-waving arguments implying that any outer faint structure might not be fundamental dynamically - based on lack of star formation there and the small number of crossing times at such distances - but we certainly need to check, and in fact very deep imaging has shown some surprising structures.

The original definition of classification criteria in blue light also has implications for use on galaxies at substantial redshifts, where we typically view in the emitted ultraviolet. Recent surveys in the UV and near-IR have led to the notion of a "morphological K-correction", a shift in stage of the morphological classification due to changes in observed wavelength. This becomes especially important if we want to compare populations of galaxies at very different redshifts, so that different sets of galaxies are seen at different emitted wavelengths. As a specific example of how galaxies can change in appearance, here is a multiwavelength series of images of M81 = NGC 3031. Some of the same effects can be seen by comparing (observed) optical and near-infrared structures of faint galaxies, such as this example from WFPC2 and NICMOS imaging in the Hubble Deep Field . These are, left to right: a ROSAT image, from data archived at HEASARC; a color-mapped UV image from GALEX; an optical color image, from data provided by G.

Bothun; a near-IR image from the 2MASS atlas [Atlas Image obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the NSF], and a 70-μ image from Spitzer. Comparing these images, starlight from the general population is important from the UV to the near-IR. Only recent star formation is important in the mid-UV, with the bulge vanishing. A more subtle shift occurs between optical and near-IR. Much of the bulge light can now be associated with a broad barlike oval distortion, and the spiral arms are now seen mostly by the light of red giants - not only are they smoother, but the ridge lines of the arms don't quite match those traced by young stars in the opti cal and UV. These young tracers tell where the density wave has driven cloud collapse, which may have a delay from the deepest potential perturbation and may not match the smoothed potential traced by the older stars.

Thought question: What kinds of galaxy classifications would have arisen if we had first encountered galaxies via the UV, or in the near-infrared? How about observing from downtown Rio or the surface of Pluto? The earliest schemes were purely descriptive, dating to times before galaxies and gaseous nebulae could be distinguished. An example is Wolf's system from 1908, shown in Fig 1 of Sandage in SSS9 : Already sequences of disk systems with changes in inclination were recognized; Curtis used photographs of such systems to study the distribution of absorbing matter in the disks of spirals even before their nature was known. The first overall scheme (which remains in widespread use) was that of Hubble. This system distinguishes ellipticals, spirals with and without bars, and irregulars. The classification criteria are: Apparent shape of ellipticals. Bulge strength and arm characteristics for spirals (arm characteristics through clumpiness or resolution most important). Spiral arms can sometimes be defined as much by dust as by blue stars. Hubble types can be conveniently arranged in the famous "Tuning Fork" diagram. The schematic example is taken from Realm of the Nebulae, as reprinted by Sandage 1975 .

Ray White has posted a tuning-fork diagram using DSS images . Here is a version including some additional deeper CCD data I had lying around: and one uising images from the Hubble telecope itself: ` The "definitive" version of the Hubble classification was set out in Sandage's article in Galaxies and the Universe. Compared to Hubble's original conception, this version adds the S0 (lenticular) class between ellipticals and spirals. Hubble hypothesized such an intermediate class, but it was only recognized later. Galaxies are often called early (E and S0) or late (Sb,Sc, Irr) in type, a remnant of early notions that galaxies physically evolve along the Hubble sequence. Unfortunately, this nomenclature is opposite to that of the dominant stellar population in these types, and to the early-late nomenclature in the Yerkes classification. Classification of an elliptical galaxy image is straightforward, because there is so little structure present. Types E0-E7 are recognized, where the number gives the projected axial ratio.

Specifically, an E0 galaxy appears circular (like M87), and in general for axial ratio b/a the number is 10(1-b/a). True ellipticals do not appear more flattened than E7, probably because there is a stability limit for nonrotating systems near this shape. Note that even the ellipticity is not completely well-defined, as many ellipticals have changes in ellipticity with radius or isophotal twists. Most appear to be triaxial systems. Spirals are divided into ordinary (S) and barred (SB) families, with a stage from S(B)a ("early" types, large bulge, tightly wound and fairly smooth arms) to S(B)c ("late" spiral, small bulge, loosely would arms, arms very textured with star-forming regions). As noted in the Hubble atlas, some spirals have arms patterns defined more by the dust lanes than by starlight per se. NGC 253 is a prominent example. For much of its extent, I find the foreground galaxy in NGC 3314 to be a personal favorite of this kind.

This classification has gained such currency because (1) it is simple and thus accomodates the great majority of bright galaxies and (2) it is our great fortune that it correlates well with astrophysical quantities like bulge/disk ratio, gas content, star-forming properties, spectrum, chemical composition of ISM, etc. This is especially nice since the classification was originally designed solely to describe galaxies' appearance on photographs! Several refinements toward a continuous sequence have been made. Sandage not only explicitly recognized transition S0 galaxies, but found subtypes depending on the amount of dust in the disk plane (and strength of the disk) denoted S0-, S0,S0+ as in the Hubble Atlas. S0 systems pose some interesting problems of recognition when not seen close to edge-on, since they are tough to tell from ellipticals with extended envelopes. de Vaucouleurs introduced several important features: (1) continuity of barred and nonbarred spirals (through mixed types SA-SAB-SB) (2) continuity of spirals into irregulars Sc-Scd-Sd-Sdm-Sm-Im (m means Magellanic, the LMC being the prototype).

(3) description of the spiral structure is paramount, rather than some resolution estimate. This makes classification less sensitive to an observer's allowance for seeing or equipment changes. He introduced classes depending on whether the inner structure is ringlike or purely spiral, with mixed types recognized: (r),(rs),(sr),(s). The stage along the sequence still runs a-c (adding, in fact, stages d and m - for Magellanic types - out beyond Sc) as in the original Hubble class. For tabulation in the Reference Catalogs and convenient numerical use, a type T is often used which is -1 for S0, 0 for S0/a, on through 5 for Sc and 9 for Im. More negative values denotes various subsets of ellipticals with or without envelopes. The article by de Vaucouleurs in Handbuch der Physik (1959, 53, 275) sets out his detailed classification principles with photographic examples. (4) the notion of a ``classification volume" rather than a one dimensional sequence. The classification volume is depicted in this sketch, most recently reproduced by Combes and Buta (1996, Fund. Cosmic Phys.

17, 95) and posted by permission of Ron Buta: The artist's conception below shows how the SA/SB and r/s classes interact at stage Sb (from Fig.1 of the RC1, likewise scanned in Combes and Buta (1996) and reposted courtesy of Ron Buta). A typical classification in the de Vaucouleurs system looks like SAB(rs)bc (in fact, this is the alleged type of our own galaxy). (5) Recognition of outer ring (prepended R) and pseudoring - denoted by initial (R) - structures. van den Bergh recognized an additional family of smooth-armed ("anemic") spirals, considering now that there exist parallel sequences S0,A,S. Thus the tuning fork becomes a trident (see Fig. 5-8 of Mihalas and Binney). These are most common in pretty rich clusters, but don't seem to be numerous for later Hubble types (small bulge:disk ratios), and the question immediately presents itself as to whether anemic spirals are waypoints between "normal" spirals and S0 systems.

He has also introduced the luminosity class, ranging from I-V, which is based on the degree of order in the spiral pattern. Sc I galaxies have very long organized arms, Sc Vs have very poorly organized patterns. These classes do correlate with absolute magnitude, although not as tightly as first hoped. One still occasionally reads of Sc I spirals as the brightest such objects, used as rough standard candles. Even with the luminosity class from arm structure, the appearance of a spiral is not a reliable guide to its linear size (something that David Block stressed years ago in an atlas published at Fort Hare, South Africa). Pretty similar-looking spirals can often differ in size by a factor 5 - M81 is a pretty small spiral compared to some of the ones in Virgo, and NGC 309, UGC 2885, NGC 3312, and NGC 6872 are huge (the largest extending to observed diameters more than 250 kpc for H0=50). van den Bergh has described many classification issues, morphological and quantitative, in Galaxy Morphology and Classification (Cambridge, 1998).

The Yerkes (or Morgan) scheme uses strictly the relative prominence of disk and bulge (the Hubble classification includes arm structure) - these are generally correlated, but not uniquely. This classification uses a form factor E,S,B, or D (for symmetric but non-E or S systems) and inclination class 1-7 (7 most elongated) plus a spectroscopic type corresponding to the nearest stellar equivalent to the spectroscopic appearance of a typical galaxy of similar morphological structure (confused yet?). This ranges from a-k, so Yerkes types look like Egk1, Iaf3, Sfg7, Bg4. The spectroscopic class is based on visual inspection of the image even though it looks like a spectral type. The system was defined and illustrated in PASP 70,364 (1958) , ApJ 135,1 (1962), ApJ 142, 1364 (1965), and AJ 76,1000.

The types most used from this system are some of the outliers: N-galaxies whose light is dominated by an unresolved nucleus, and cD galaxies for supergiant (spectroscopic nomenclature c) galaxies with extended envelopes. As noted in van den Bergh's book, the concentration changes little over the Hubble type range E-S0-Sa, and changes a great deal within the Sc class, graphically demonstrating that physical properties for any kind of purely morphological classification may have a highly nonlinear mapping to that classification's criteria, even when we are fortunate enough for there to be a useful mapping at all. Classifications of this kind, based on light concentration, have received renewed interest in the context of high-redshift galaxies, and with the recognition that they can be made robust even for poorly resolved systems by appropriate modelling. Vorontsov-Velyaminov produced a purely descriptive scheme that incorporates peculiarities such as three-armed spirals and gamma-forms. This accomodates most of the oddballs that the Hubble scheme doesn't, but does not yet allow a continuous sequence. It was illustrated in vol. 1 of the MCG.

This points up various attitudes toward galaxies that don't really fit in systems like the Hubble classification - is it right to force them into one of the bins? How peculiar is peculiar? As transient phenomena, should interaction-induced distortions be dignified with a place in the scheme et all? (De Vaucouleurs wrote several times that we don't have a special model name for the prodicts of an automotiove collision. But then cars aren't self-gravitating systems). Spiral structure has been divided into grand-design and flocculent types, depending on the level of organization. The Elmegreens (1982 MNRAS 201, 1021; 1987, ApJ 314, 3) use an index that distinguished these, still on a purely visual level. The arm class ranges from 1 ("chaotic, fragmented, unsymmetric spiral arms") to 12 ("two long symmetric arms dominating the optical disk") Such distinctions have been long noted (for example in the Hubble Atlas and in part in van den Bergh's luminosity classes), but interest in disk dynamics eventually provoked a separate numerical estimate of arm organization.

Some galaxies fall outside the commonly recognized sequences. Many of these are "train wrecks", transient forms produced by the interaction or merger of galaxies. These forms include ring (not ringed) galaxies, polar rings, shells, and systems with tails, as well as double nuclei and highly asymmetric galaxies. Possibly related are what Hubble called Irr II and de Vaucouleurs calls I0 systems. The type example is M82; a galaxy with early-type stellar spectrum, but no particular spiral structure and amorphous appearance. These are almost always found in dense environments and appear to result from interaction-induced bursts of star formation. There have been numerous papers purporting to give a ``true" physical explanation for the Hubble sequence. Their authors are, to my mind, missing the point. Classification must begin as a descriptive process; the desired physical insight is a rather different thing. Basic references on classification: for the Hubble system, see the Hubble Atlas, Revised Shapley Ames Catalog, Sandage-Bedke NASA atlas, and Carnegie Atlas of Galaxies.

Luminosity classification of spirals was described by van den Bergh in ApJ 131, 215 (1960) and ApJ 131, 558 (1960); the NASA atlas includes many examples. de Vaucouleurs (1977, Yale conference p. 43) discusses comparison of classification with quantitative parameters. Dwarf galaxies were not included in these original schemes; see Sandage and Binggeli 1984 (AJ 89, 919). Many useful references are given in van den Bergh's review (JRAS Canada 69, 57, 1975) and in Sandage, SSS vol 9. Dwarfs can be classified based on whether a distinct nucleus is present and on whether the structure is spheroidal or more irregular. Some fall in the low-surface-brightness (LSB) category, though this also includes very large and massive systems; our understanding of LSB galaxies and how they relate to other kinds is still in its infancy.

Classification on these systems is fairly robust; experienced observers generally agree on a galaxy's Hubble type to a level DT = 0.7 on the scale where the range from, say, Sa to Sab is one unit (and automated systems can be trained to a similar level). So what is this system "telling" us? The Hubble type (or T) can be shown to correlate with: bulge/disk luminosity ratio relative H I content M(H I)/L(B) mass concentration Hubble Sequence Answer Key - HelpTeaching.com (Only the test content will print) Hubble Sequence Answer Key Use the diagram below to answer the questions. 1. Young galaxies are located on the left side of the diagram and older galaxies fall on the right side of the diagram. True S0 is classified as a spiral galaxy. True The Hubble sequence classifies galaxies based upon their age Which type of the galaxies listed below is not shown on the diagram? spiral Which description best fits a SBc galaxy? spiral with tightly wound arms spiral with loosely wound arms barred spiral with tightly wound arms barred spiral with loosely wound arms 6.

From right to left, the Hubble sequence shows elliptical, lenticular, and spiral galaxies spiral, lenticular, and elliptical galaxies. elliptical, irregular, and spiral galaxies spiral, irregular, and elliptical galaxies. 7. The major difference between an E3 galaxy and an E7 galaxy is E3 is more elliptical in shape than E7 E7 is more elliptical in shape than E3 E3 has barred spiral arms and E7 does not E7 has barred spiral arms and E3 does not 8. M95 is classified as a SBb galaxy. Andromeda galaxy is classified as Sb. The difference in classification indicates which of the following? M95 is a barred galaxy and Andromeda is not Andromeda is a barred galaxy and M95 is not M95 has more tightly wound spiral arms than Andromeda Andromeda has more tightly wound spiral arms than M95 9. M86 is a slightly elliptical galaxy. Based on the diagram, which would be the best classification of M86?

E0 The Hubble Tuning Fork - Classification of Galaxies | ESA/Hubble A Danish Communication Strategy The Hubble Tuning Fork - Classification of Galaxies Galaxies are very important fundamental building blocks of the Universe. Some are simple, while others are very complex in structure. As one of the first steps towards a coherent theory of galaxy evolution, the American astronomer Edwin Hubble , developed a classification scheme of galaxies in 1926. Although this scheme, also known as the Hubble tuning fork diagram, is now considered somewhat too simple, the basic ideas still hold. The diagram is roughly divided into two parts: elliptical galaxies (ellipticals) and spiral galaxies (spirals). Hubble gave the ellipticals numbers from zero to seven, which characterize the ellipticity of the galaxy - "E0" is almost round, "E7" is very elliptical. The spirals were assigned letters from "a" to "c," which characterize the compactness of their spiral arms. "Sa" spirals, for example, are tightly wound whereas "Sc" spirals are more loosely wound.

Also it is worth noting that the sizes of the round central regions in spirals - the so-called bulges - increase in size the more tightly the spiral arms are wound. There are indications pointing to a very close connection between the bulges of certain galaxies (Hubble types "S0", "Sa" and "Sb") and elliptical galaxies. They may very well be similar objects. In fact the spiral galaxies are sub-divided into two groups - normal spirals and barred spirals. The most important difference between these two groups is the bar of stars that runs through the central bulge in barred spirals. The spiral arms in barred spirals usually start at the end of the bar instead of from the bulge. Barred spirals have a "B" in their classification. An "SBc" is thus a loosely wound barred spiral galaxy. "S0," or lenticular galaxies, are in the transition zones between ellipticals and spirals and bridge these two types. Hubble found that some galaxies are difficult to put in the context of the tuning fork diagram.

Those include irregular galaxies which have odd shapes, dwarf galaxies which are very small, and giant elliptical galaxies which are very large elliptical galaxies residing in the centers of some clusters of galaxies. For a time the Hubble tuning fork was thought to be an evolutionary sequence - that galaxies might evolve from one type to another progressing from left to right across the tuning-fork diagram. Hence "Sa" and "SBa" galaxies were called "early-type," while "Sc" and "SBc" were called "late-type." Astronomers still use this nomenclature today, though the initial concept was later found to be an over-simplification. Galaxy evolution is a far more complex process than Hubble imagined, involving the conditions of the galaxy's initial collapse, collisions with other galaxies, and the ebb and flow of internal star birth. Credit: Classification of Galaxies long version Classification of Galaxies Our galaxy is one of billions of galaxies populating the universe. It would be the height of presumption to think that we are the only living things in that enormous immensity -- Werner Von Braun Become familiar with the Hubble classification system. Become familiar with the similarities and differences between types of galaxies. Look at the differences between the modern and early universe.

Introduction In the early part of the 20th century, Edwin Hubble began studying “spiral nebulae.” Distance measurements using the period-luminosity relationship for Cepheid variables would eventually show these “nebulae” were in fact other galaxies outside our own. Hubble came to recognize that there are several types of galaxies based on their visual appearance (morphology). He expanded his studies to include all types of galaxies, and was eventually able to show that the universe is expanding (Hubble’s law). To help his studies, he also developed a morphological classification system for galaxies. Despite modifications and refinements over the years reflecting our increasing understanding of these beautiful stellar systems, the basic Hubble classification still remains in widespread use today. In Hubble’s classification, galaxies are divided into 5 basic groups: elliptical, lenticular or S0, spiral, barred spiral, and irregular. Elliptical galaxies generlly lack any overal structure except for their general shape. Spirals and barred spirals have a distinct disk and bulge, and get their name from the fact that there appear to be spiral arms in the disk.

Lenticulars appear very similar to the ellipticals but actually have a distinct bulge and disk. In most cases, the disk is very hard to make out in optical images. Irregular is the catch-all for everything else. Each group is further divided based on characteristics within the group: Ellipticals are numbered 0  7 based on their “ellipticity” e (note this is 10 times the eccentricity) e=10(major axis  minor axis)/major axis An E0 would be round, an E7 cigar shaped Spirals and barred spirals are labeled a b or c based on a combination of the relative size of the bulge and wrap of the arms a’s have large bulges and tightly wrapped arms c’s have small bulges and loosely wrapped arms Barred spirals have a round or oval bulge with a nearly rectangular bar across it. The spiral arms attach to the bar rather than the bulge. An Sa would be a galaxy with a large bulge and tightly wrapped arms and an SBc would have a small, oval bulge with arms that attach to the end of the bar and spread out away from the center.

Lenticulars generally appear round like an E0 but have a distinct bulge and/or a disk like a spiral. Irregulars are not easily classified into any of these groups. Many irregulars appear to be interacting with other galaxies. In addition, there are peculiar galaxies. These are classifiable as a particular type but have something strange about them. For example, it may be classifiable as an edge-on type Sc but the spiral disk may be warped, or as a E4 but have a distinct blob of gas near one edge. The first atlas of peculiar galaxies was compiled by Halton C. Arp in 1966. In many cases he classified galaxies as peculiar because there was an unresolved companion, so it looked like there was a blob on the side of the galaxy. Overal there are 16 different classes for galaxies in this system. It works fairly well for the nearby bright galaxies Hubble was studying. However, as astronomy developed, especially at non-optical wavelengths, the need for more catagories arose. Several other classification systems were developed, but the most popualr was developed by Gerard de Vaucouleurs in 1959.

He basically expanded the Hubble classification system allowing for inermediate classes such as Sab, and adding extra notations such as + and - signs or extra letters like m to indicate things like the the strength of the bar, disk structure in lenticulars, rings in sprirals, and different structures in irregulars. Using this classification system, there are hudreds of different types of galaxies possible. This is the system generally used if the Hubble classification system in insufficient. In this activity, you use the Hubble classification system to develop a method for classifying galaxies from a set of pre-classified galaxies. Then you'll classify several mystery galaxies. After that, you will draw some conclusions about the history of several types of galaxies based on the color images. If you want more information on the objects or on any of the catalogues and classification systems, is an excellent place to start. Part 1:The Classification System You need the checklist and 9 images with known classification. Spread the images out in front of you in the same order as the checklist. Based on your overall view of these images, answer the questions below.

How do you tell the difference between the dense bulge at the center of a lenticular and a photographic/ printing effect making the center of an elliptical galaxy look like a dense center? How do you determine where the edge of the galaxy is in order to measure its diameter (consider especially the E0 and SBb galaxies)? How do you tell if a galaxy has a disk if it is (a) face on and (b) edge on? Now you are going to develop a checklist to help you classify galaxies. Answer the following questions for each of the pictures. If the answer is yes, place a check in the appropriate space on the checklist and leave it blank if the answer is no. If it asks for a number, fill in the number in the appropriate space. This section will determine the basic type: elliptical, lenticular, spiral, barred spiral or irregular. Is there a distinct bulge at the center? Is the bulge round? Is there a distinct disk? Are there spiral arms visible? Do the spiral arms attach directly to the bulge?

(the other option would be attaching to a bar that cuts across the bulge) If there are spiral arms, rank the wrap of the arms on a scale of 1 to 10 where 1 is very tightly wrapped (circles back around the galaxy) and 10 is very loose (curves away in an arc) Is there any structure other than a bulge visible or anything unusual looking? This section will determine the number or letter for the elliptical or spiral. You can skip these questions if it is an irregular or lenticular galaxy. Measure the major axis of the galaxy. If it appears to be a round elliptical, take the longest of several measurements. If it is a spiral-type and appears round, take the average of several measurements. Enter this as x in the checklist. Measure the minor axis if it is an elliptical (perpendicular to the direction you measured in step 8), or the diameter of the bulge if it is a spiral or barred spiral (parallel to the direction you measured in step 8). Enter this under the y column in the checklist. Calculate the ellipticity e if it is an elliptical, or the bulge  to  disk b/d ratio for a spiral.

Enter this in the last column. Once you have completed the table, you will have a check list to determine the galaxy type based on the known galaxies. Answer the following questions before going onto part 2. What would the checklist look like for an E6 galaxy? What would the checklist look like for an SBa peculiar galaxy? How would you classify a galaxy with loosely wrapped arms but a large round bulge? Why? Part 2: Unknown galaxies Your GSI will give you a list for the unknown galaxies. You will need to find the images of the galaxies on your list, then determine their classification as outlined below. Begin by recording their names in table 1. In the next column, record which checklist items from numbers 1 - 5 and 7 you answered yes to for each picture, and the rank for 6 if it is a spiral galaxy. If it is an elliptical or spiral galaxy, measure x and y and calculate e or b/d (checklist items 8 - 10), and record the values in the next 3 columns. Any additional comments should go in the second to last column.

This is a good place to make notes about how you made your final determination of the galaxy type, especially for peculiars and boarder-line galaxies (e.g. an elliptical with e=4.5 or a spiral with a large bulge and loose arms) Finally, determine the classification and record it in the last column. Questions What problems did you run into classifying the galaxies? How confident are you in your classification: do you think your classification would match if you look up your galaxies in a catalogue? Explain. Part 3: Colors and what they tell us Join with another group who had a different set of galaxy images and sort your galaxy images into piles according to the basic types. Look at your spiral and barred spiral galaxies. What color do these galaxies tend to be (some areas may be saturated and look white, so look closer to the edge)? If different regions are different colors note that. Are there features like HII regions, dust lanes, planetary nebulae or anything else? Where? Look at the images of the elliptical galaxies What color do these galaxies tend to be (some areas may be saturated and look white, so look closer to the edge)? If different regions are different colors note that.

Are there features like HII regions, dust lanes, planetary nebulae or anything else? Where? Look at the images of the lenticular galaxies What color do these galaxies tend to be (some areas may be saturated and look white, so look closer to the edge)? If different regions are different colors note that. Are there features like HII regions, dust lanes, planetary nebulae or anything else? Where? Look at the images of the irregular galaxies What color do these galaxies tend to be (some areas may be saturated and look white, so look closer to the edge)? If different regions are different colors note that. Are there features like HII regions, dust lanes, planetary nebulae or anything else? Where? Pull out all the images of galaxies you classified as peculiar. What type(s) do the peculiars fall into (elliptical, lenticular or spiral/barred spiral)? Is one type more common? Which one? Is there anything all peculiars of the same basic type have in common that is not common to the other galaxies of the same basic type? What is it? If you have peculiars of more than one basic type, list commonalities for all types.

Based on your answers to the above 10 questions, answer the following: Which type(s) of galaxy tends to have old stars? How did you determine this? Which type(s) of galaxies tend to have ongoing star formation? How did you determine this? What sort of event can trigger galaxy-wide star formation? Explain how you determined this event is a trigger. Part 4: The Hubble UDF Look at the Hubble Ultra Deep Field (UDF) image segment (available here if you want to see the original) Note there are no stars, only galaxies in this image. Light from most of the galaxies in this image left when the universe was about 1 billion years old, so they provide the earliest snapshot yet of what galaxies were like when the universe was young. Estimate the number of galaxies in the image. Describe how you estimated the number. Estimate the number of galaxies you can classify as spiral, elliptical or lenticular. What fraction of the total number of galaxies is this? Estimate the number of irregular galaxies. What fraction of the total number of galaxies is this? Is the Hubble classification system useful for the UDF? How were galaxies in the early universe different from galaxies in the modern universe?

Last modified: 7/12/05 Hubble explores the origins of modern galaxies | ESA/Hubble Hubble explores the origins of modern galaxies Astronomers see true shapes of galaxies 11 billion years back in time 15 August 2013 Astronomers have used observations from Hubble’s CANDELS survey to explore the sizes, shapes, and colours of distant galaxies over the last 80% of the Universe’s history. In the Universe today galaxies come in a variety of different forms, and are classified via a system known as the Hubble Sequence — and it turns out that this sequence was already in place as early as 11 billion years ago. The Hubble Sequence classifies galaxies according to their morphology and star-forming activity, organising them into a cosmic zoo of spiral, elliptical, and irregular shapes with whirling arms, fuzzy haloes and bright central bulges. Two main types of galaxy are identified in this sequence: elliptical and spiral, with a third type, lenticular , settling somewhere between the two. This accurately describes what we see in the region of space around us, but how does galaxy morphology change as we look further back in time, to when the Universe was very young?

"This is a key question: when and over what timescale did the Hubble Sequence form?" says BoMee Lee of the University of Massachusetts, USA, lead author of a new paper exploring the sequence. "To do this you need to peer at distant galaxies and compare them to their closer relatives, to see if they too can be described in the same way." The astronomers used Hubble to look 11 billion years back in time to when the Universe was very young, exploring the anatomy of distant galaxies. While it was known that the Hubble Sequence holds true as far back as around 8 billion years ago [1] , these new observations push a further 2.5 billion years back in cosmic time, covering a huge 80% of the past history of the Universe. Previous studies had also reached into this epoch of the cosmos to study lower-mass galaxies, but none had conclusively also looked at large, mature galaxies like the Milky Way. The new CANDELS observations confirm that all galaxies this far back — big and small alike — fit into the different classifications of the sequence.

"This is the only comprehensive study to date of the visual appearance of the large, massive galaxies that existed so far back in time," says co-author Arjen van der Wel of the Max Planck Institute for Astronomy in Heidelberg, Germany. "The galaxies look remarkably mature, which is not predicted by galaxy formation models to be the case that early on in the history of the Universe." The galaxies at these earlier times appear to be split between blue star-forming galaxies with a complex structure — including discs, bulges, and messy clumps — and massive red galaxies that are no longer forming stars, as seen in the nearby Universe [2] . Galaxies as massive as the Milky Way or more are rather rare in the young Universe. This scarcity has prevented previous studies from being able to gather a large enough sample of mature galaxies to properly describe their characteristics. What was needed was a systematic set of observations such as those from Hubble's CANDELS survey, which was large enough to allow the astronomers to analyse a larger number of these galaxies consistently, and in detail [3] .

With Hubble's Wide Field Camera 3 (WFC3), the astronomers were able to observe in the infrared part of the spectrum to see how the galaxies appeared in their visible rest-frame [4] , which is easier to compare with galaxies in our neighbourhood. "The huge CANDELS dataset was a great resource for us to use in order to consistently study ancient galaxies in the early Universe," concludes Lee. "And the resolution and sensitivity of Hubble's WFC3 is second to none in the infrared wavelengths needed to carry out this study. The Hubble Sequence underpins a lot of what we know about how galaxies form and evolve — finding it to be in place this far back is a significant discovery." Notes [1] Previous studies have looked at the proportions of the different galaxy types back in time ( heic1002 ). The mix of spiral, elliptical, lenticular and peculiar galaxies is different from today, with a great many more peculiars in the distant Universe than we see nearby. [2] In a related recent paper , Alice Mortlock and collaborators took a different but complementary approach by classifying these distant galaxies by visual inspection.

They found that the types of galaxies we see in the Hubble Sequence are well defined in terms of colour, structure, and star formation rates at very large distances from us, but that their morphology is still developing. While the morphology of a galaxy may be the final property to settle, the fundamentals of the Hubble Sequence are set much earlier on. [3] CANDELS, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey , is the largest project in the history of Hubble, with 902 assigned orbits of observing time. It is being carried out with two cameras on board Hubble – WFC3 and ACS – and aims to explore galactic evolution in the early Universe, and the very first seeds of cosmic structure at less than one billion years after the Big Bang. [4] Previous studies of this period of cosmic history were inconclusive as they were limited to visible light, showing only the redshifted ultraviolet emission of the galaxies, which highlights star formation. As this star formation dominated the observations, the galaxies appeared to be clumpy and messy, with no resemblance to the galaxy shapes we see around us today.

By pushing into the infrared part of the spectrum the astronomers could observe how these distant galaxies appear in their visible rest frame (which is now redshifted). Notes for editors The Hubble Space Telescope is a project of international cooperation between ESA and NASA. These results are described in a paper entitled "CANDELS: The correlation between galaxy morphology and star formation activity as Z~2", to appear in The Astrophysical Journal. [1] The international team of astronomers in this study consists of B. Lee (University of Massachusetts, USA), M. Giavalisco (University of Massachusetts, USA), C. C. Williams (University of Massachusetts, USA), Y. Guo (University of California, USA), J. Lotz (Space Telescope Science Institute, Baltimore, USA), A. van der Wel (Max Planck Institute for Astronomy, Heidelberg, Germany), H. C. Ferguson (Space Telescope Science Institute, Baltimore, USA), S. M Faber (University of California, USA), A. Koekemoer (Space Telescope Science Institute, Baltimore, USA), N. Grogin (Space Telescope Science Institute, Baltimore, USA), D. Kocevski (University of Kentucky, USA), C. J.

Conselice (University of Nottingham, UK), S. Wuyts (Max Planck Institute for Extraterrestrial Physics, Garching, Germany), A. Dekel (The Hebrew University, Israel), J. Kartaltepe (NOAO-Tuscon, Arizona, USA), E. F. Bell (University of Michigan, USA). More information

Question: What are classified using the Hubble Sequence?
Answer: Galaxies